AIPS NRAO AIPS HELP file for FIXWT in 31DEC18



As of Tue Apr 24 8:25:45 2018


FIXWT: Modify weights to reflect amplitude scatter of data.

INPUTS

INNAME                             Input UV file name (name)
INCLASS                            Input UV file name (class)
INSEQ              0.0      9999.0 Input UV file name (seq. #)
INDISK             0.0         9.0 Input UV file disk drive #
OUTNAME                            Output UV file name (name)
OUTCLASS                           Output UV file name (class)
OUTSEQ            -1.0     9999.0  Output UV file name (seq. #)
OUTDISK            0.0        9.0  Output UV file disk unit #.
DOUVCOMP                           Write compressed data?
FLAGVER                            Flag table to apply.
BCHAN              0.0      4096.0 Begin channel 0 => 1
ECHAN              0.0             End channel
SOLINT             0.0             Averaging interval
DOALL             -1.0         1.0 > 0 -> find overall scaling
                                   factor for weights
CPARM                              Limit controls
BADDISK                            Disk no. not to use for
                                      scratch files.

HELP SECTION

FIXWT
Task:  AIPS assumes that the weights given to UV data points are the
       inverse square of the estimated standard deviation of the
       amplitudes due to thermal (and other) noise. Unfortunately, not
       all input data comforms to this requirement.  FIXWT runs a
       filter of width SOLINT minutes over the data and calculates
       new weights for each antenna as function of time from the
       scatter of the visbilities.  The data must be in time order,
       but the baselines at any given time may be in any order.

       Subarray ID is ignored, so multiple subarrays may exist only if
       they are separated in time.  Multiple sources or Frequency IDs
       cause an integration to break.

       NOTE: this task does NOT apply flagging or calibration tables
       to the input UV data.  Run SPLIT first if that operation is
       desired.
Adverbs:
  INNAME.....Input UV file name (name).    Standard defaults.
  INCLASS....Input UV file name (class).   Standard defaults.
  INSEQ......Input UV file name (seq. #).  0 -> highest.
  INDISK.....Disk drive # of input UV file.  0 -> any.
             Note that the input file must be TB sorted
  OUTNAME....Output UV file name (name).     Standard defaults.
  OUTCLASS...Output UV file name (class).    Standard defaults.
  OUTSEQ.....Output UV file name (seq. #).   0 => highest unique
  OUTDISK....Disk drive # of output UV file. 0 => highest w room
  DOUVCOMP...If DOUVCOMP > 0 then the output data (and the
             scratch files) will be compressed.
  FLAGVER....specifies the version of the flagging table to be applied.
              0 => highest numbered table.
             <0 => no flagging to be applied.
             Flags are applied when reading data to determine weights
             but are not applied when reading the data to merge the
             weights in with the visibilities.
  BCHAN......Begin channel number. 0 -> 1
  ECHAN......End channel number.  0 -> highest
             The channels are separate samples of the amplitude; all
             channels will be written to the output.
  SOLINT.....Averaging interval (minutes).  0 -> 1.  You must have at
             least 5 samples to compute an rms - these may be multiple
             spectral channels or multiple times.
  DOALL......IF <= 0, the weights are found by antenna for each SOLINT
             (broken if source or FQ changes) and applied in that
             form.  If > 0, the weights are found in the same way, but
             are then compared to the existing weights to determine an
             over-all correction factor to be applied to all weights.
  CPARM......Values can get out of hand here so limits things as
             (1) Minimum rms allowed (Jy): 0 -> 0.
             (2) Maximum rms allowed (Jy): 0 -> very large
             (3) Minimum antenna weight allowed: 0 -> 0
             (4) Maximum antenna weight allowed: 0 -> very large
             Note a baseline weight is a product of 2 antenna weights
             and should be 1 / sigma**2.
             (5) When DOALL is false, the weights are found from the
             rms averaging over, in principle, multiple records.
             These records may have different weights in averaged
             data, representing different integration times.  If
             CPARM(5) <= 0, then the output weights will be 1/rms**2
             ignoring differences in integration time.  You may have
             the task down-weight samples that have lower weights,
             setting the actual weight to
                   (WTout) * (WTin/Max(WTin))**CPARM(5)
             where WTout is the weight based on the rms, WTin is the
             individual sample's weight on input and Max(WTin) is the
             maximum of WTin over the whole data set.  If the input
             weights are of the "old" type representing a simple count
             of 10-second records, set CPARM(5)=1.  If the input data
             are averaged but with weights representing an estimate of
             system temperature, set CPARM(5) to something between 0
             and 1, e.g. 1/3.
  BADDISK....Disk numbers on which scratch files are not to be
             placed.

EXPLAIN SECTION

FIXWT:  Task to fix uv data weights
Documentor:  Chris Flatters
Related Programs: almost all programs operating on UV data

AIPS assumes that the weight given to a uv data point is the
statistically correct weight of unity divided by the estimated
variance due to noise.  Various data input sources, including the VLA
and the VLBA, do not always follow this convention, however. This can
lead to a number of problems ranging from the relatively trivial
problem of incorrectly plotted error bars in uv plots to more serious
problems involving calibration and imaging, particularly where data
from more than one array are being combined.

FIXWT deals with this problem by copying its input file, assigning new
weights, based on the actual standard deviations of the visibilities
in the file.  It assumes that the same weights may be assigned to all
spectral channels, but sets different weights for IFs and
polarizations.  The weights are determined independently for each
SOLINT integration time on the assumption that the rms squared on a
given baseline is the product of the separate antenna rms's.  The
weights are not assumed to be continuous over changes of source or
frequency ID or between SOLINT intervals.  Data points will be flagged
by setting their weights to zero in ranges where the standard
deviation of the data cannot be determined due to insufficient data.

Increasing SOLINT will give a smoother estimate of the noise.

Fully flagged visibilities will not appear in the output file so INDXR
should be run on the output file if the input was a multi-source file.
This step can be omitted, however, if the number of visibilities in
the output file is the same as that in the output file.



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